Primary structure of chicken liver dihydrofolate reductase

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Abstract
The complete covalent structure of dihydrofolate reductase from chicken liver is described. The S-carboxymethylated protein was subjected to cleavage by cyanogen bromide which produced 5 fragments. Fragment CB2 contained an internal homoserine residue which was not cleaved by cyanogen bromide. Sequences and ordering of the cyanogen bromide fragments were established by automated sequencer analyses of the fragments and from smaller peptides generated by proteolysis with trypsin and staphylococcal protease. The covalent structure of the single polypeptide chain comprises 189 residues of MW 21,651. The chicken liver enzyme is homologous to that from [mouse lymphoid tumor] L 1210 cells and shows regions of homology to dihydrofolate reductases from Streptococcus faecium, Escherichia coli and Lactobacillus casei. These homologous regions in the chicken liver enzyme are primarily related to conserved amino acid residues implicated in the binding of NADPH and methotrexate by bacterial dihydrofolate reductases.